The present invention relates to an epitaxial wafer comprising a GaAs.sub.1-x P.sub.x single crystal layer that is grown on a GaAs or GaP single crystal substrate, wherein a GaAs.sub.1-x P.sub.x layer in which x is gradually varied from 0 before being fixed is grown between the single crystal substrate and the GaAs.sub.1-x P.sub.x layer. The present invention also relates to a process for producing the described epitaxial wafer.
A wafer comprising a GaAs.sub.1-x P.sub.x single crystal film that is epitaxially grown on a GaP single crystal substrate is widely used as a material for light-emitting diodes after having Zn diffused thereinto to form a pn junction. In general, nitrogen (N) is doped into the GaAs.sub.1-x P.sub.x layer to induce an isoelectronic trap, thereby increasing the luminous efficiency. The wavelength of light emitted from a light-emitting diode is determined by the composition x: x=0.9 is employed to emit yellow light, x=0.75 for orange light, and x=0.65 for red light.
If the quality of GaAs.sub.1-x P.sub.x crystal is inadequate, a non-luminescent center is induced in the resulting light-emitting diode. Therefore, GaAs.sub.1-x P.sub.x crystal must have excellent quality in order to provide a high-luminance light-emitting diode. For example, if a fixed-composition layer 11 that has a fixed composition, i.e., x=0.75, is epitaxially grown directly on a GaP substrate 12, as shown in FIG. 8, the lattice mismatch cannot completely be relieved due to the lattice constant difference, so that misfit dislocations are extended into the fixed-composition layer 11 from the interface 13, resulting in the crystal quality of the layer 11 being deteriorated considerably. In order to avoid this problem, it is a conventional practice to form a varied-composition layer 22, in which the composition x is gradually varied, between a GaP substrate 23 and a fixed-composition layer 21, as shown in FIG. 9, thereby relieving the lattice mismatch between the GaP substrate 23 and the fixed-composition layer 21. Thus, misfit dislocations at the interface 25 between the GaP substrate 23 and the epitaxial layer 22 can be prevented from extending into the GaAs.sub.1-x P.sub.x fixed-composition layer 21, so that excellent crystal quality can be obtained.
In the conventional structure that is shown in FIG. 9, the rate of change in the composition of the varied-composition layer 22 is, in general, 0.02 or less per .mu.m in the direction of growth. It is known that a rapid change in the composition not only deteriorates the crystal quality of the fixed-composition layer 21 that is formed subsequently to the varied-composition layer 22, but also causes surface crystal defects, for example, hillocks, on the surface of the fixed-composition layer 21.
Even if the fixed-composition layer 21 is formed through the varied-composition layer 22, lattice mismatch is still present between the fixed-composition layer 21 and the GaP substrate 23, and it is therefore impossible to completely relieve the lattice mismatch by the varied-composition layer 22. The crystal quality of the fixed-composition layer 21 greatly depends on the method of producing the varied-composition layer 22, resulting in large variations in the luminance (light output) of a light-emitting diode produced by employing the resulting epitaxial wafer. More specifically, if the rate of change in the composition of the varied-composition layer 22 is reduced, the thickness of the varied-composition layer 22 unavoidably increases, which results in an increase in the material cost. In addition, it is impossible to improve the crystal quality of the fixed-composition layer 21 simply by reducing the rate of change in the composition of the varied-composition layer 22.